The present invention relates to a valve mechanism for a double overhead camshaft (DOHC) engine with horizontally opposed cylinders, and more particularly to a hydraulic valve-lash adjuster provided in the valve mechanism.
The DOHC engine is provided with a valve mechanism having a rocker arm pivotally mounted on the hydraulic valve-lash adjuster at an end thereof. The rocker arm is rocked by a cam engaged with the rocker arm at a central position thereof. In the horizontally opposed cylinder engine, the intake valve of each cylinder is arranged such that the axis thereof is upwardly inclined and the exhaust valve is disposed in such a manner that the axis of the valve is downwardly inclined. The hydraulic valve-lash adjusters are also inclined in the same direction of the corresponding valves, accordingly.
Japanese Patent Application Laid-Open No. 59-99012 discloses a hydraulic valve-lash adjuster in which a high pressure chamber is provided between a body and a plunger of the hydraulic valve-lash adjuster and a reservoir chamber is formed in the plunger. The reservoir chamber is tapered off toward the high pressure chamber so that bubbles produced in oil in the reservoir chamber may be guided to a remote position from an inlet of a check valve of the high pressure chamber.
In the system, if the hydraulic valve-lash adjuster is horizontally mounted or upwardly inclined, air can be prevented from entering the high pressure chamber due to the tapered reservoir chamber. However, if the hydraulic valve-lash adjuster is downwardly inclined, the high pressure chamber is located on the upper portion of the reservoir chamber and the tapered reservoir chamber is reversed. Consequently, air flows into the high pressure chamber. Accordingly, it is necessary to change the location of the high pressure chamber with respect to the reservoir chamber.
Further, there are other problems. If the clearance between the body and the plunger is large, a large amount of oil leaks from the high pressure chamber and the plunger is unstable in sliding in the body because of lateral force from the rocker arm. In order to resolve the problems, it is preferable to reduce the diameter of the plunger. However, if the diameter of the plunger is reduced, the diameter of a spring becomes small, resulting in reduction of spring force.
On the other hand, during the stop of the engine, oil in the high pressure chamber is leaked. When the engine is started, the plunger is moved by pressurized oil and by the plunger spring to rock the rocker arm through a pivot rod, thereby engaging the end of the rocker arm with the end of the valve stem. However, if the spring force is weak, the engagement of the rocker arm delays. Accordingly, the valve strikes the valve seat, causing noises.